Hydrocarbon compounds are useful for a number of purposes. In particular, hydrocarbon compounds are useful, inter alia, as fuels, solvents, degreasers, cleaning agents, and polymer precursors. The most important source of hydrocarbon compounds is petroleum crude oil. Refining of crude oil into separate hydrocarbon compound fractions is a well-known processing technique.
Crude oils range widely in their composition and physical and chemical properties. Heavy crudes are characterized by a relatively high viscosity, low API gravity, and high percentage of high boiling components (i.e., having a normal boiling point of greater than 510° C. (950° F.)).
Refined petroleum products generally have higher average hydrogen to carbon ratios on a molecular basis. Therefore, the upgrading of a petroleum refinery hydrocarbon fraction is generally classified into one of two categories: hydrogen addition and carbon rejection. Hydrogen addition is performed by processes such as hydrocracking and hydrotreating. Carbon rejection processes typically produce a stream of rejected high carbon material which may be a liquid or a solid; e.g., coke for fuel or metallurgical applications.
The higher end boiling point components, sometimes referred to as bottom-of the-barrel components, may be converted using various upstream conversion processes. In some embodiments, vacuum residua streams may be partially converted. The vacuum residua streams, however, may only be partially converted in order to prevent significant downtimes in processes downstream due to fouling and deposition of carbonaceous deposits.
Hydrocracking processes can be used to upgrade higher boiling materials within the partially converted vacuum residua by converting them into more valuable lower boiling materials. For example, a partially converted vacuum residua fed to a hydrocracking reactor may be converted to a hydrocracking reaction product. The unreacted partially converted vacuum resid may be recovered from the hydrocracking process and either removed or recycled back to the hydrocracking reactor in order to increase the overall vacuum residua conversion.
The conversion of partially converted vacuum residua in a hydrocracking reactor can depend on a variety of factors, including feedstock composition; the type of reactor used; the reaction severity, including temperature and pressure conditions; reactor space velocity; and catalyst type and performance. In particular, the reaction severity may be used to increase the conversion. However, as the reaction severity increases, side reactions may occur inside the hydrocracking reactor to produce various byproducts in the form of coke precursors, sediments (i.e., precipitated asphaltenes, and other deposits) as well as byproducts which may form a secondary liquid phase. Excessive formation of such sediments can hinder subsequent processing and can deactivate the hydrocracking catalyst by poisoning, coking, or fouling. Deactivation of the hydrocracking catalyst can not only significantly reduce the residua conversion, but can also require more frequent change-outs of expensive catalysts. Formation of a secondary liquid phase not only deactivates the hydrocracking catalyst, but also limits the maximum conversion, thereby resulting in a higher catalyst consumption, and which can defluidize ebullated-bed catalysts. This leads to formation of “hot zones” within the catalyst bed, exacerbating the formation of coke deposits, which further deactivates the hydrocracking catalyst.
Sediment formation inside the hydrocracking reactor is also a strong function of the feedstock quality. For example, asphaltenes that may be present in the partially converted vacuum residua feed to the hydrocracking reactor system are especially prone to forming sediments when subjected to severe operating conditions. Thus, separation of the asphaltenes from the partially converted vacuum residua in order to increase the conversion may be desirable.
One type of process that may be used to remove such asphaltenes from the partially converted vacuum residua feed is solvent deasphalting. For example, solvent deasphalting typically involves physically separating the lighter hydrocarbons and the heavier hydrocarbons including asphaltenes based on their relative affinities for the solvent. A light solvent, such as a C3 to C7 hydrocarbon, can be used to dissolve or suspend the lighter hydrocarbons, commonly referred to as deasphalted oil, allowing the asphaltenes to transfer into a separate phase. The two phases are then separated and the solvent is recovered. Additional information on solvent deasphalting conditions, solvents and operations may be obtained from U.S. Pat. Nos. 4,239,616; 4,440,633; 4,354,922; 4,354,928; and 4,536,283.
Several methods for integrating solvent deasphalting with hydrocracking in order to remove asphaltenes from vacuum residua are available. Such processes are disclosed in U.S. Pat. No. 7,214,308 which disclose contacting the vacuum residua feed in a solvent deasphalting system to separate the asphaltenes from deasphalted oil. The deasphalted oil and the asphaltenes are then each reacted in separate hydrocracking reactor systems.
Moderate overall vacuum residua conversions (about 65% to 70% as described in U.S. Pat. No. 7,214,308) may be achieved using such processes, as both the deasphalted oil and the asphaltenes are separately hydrocracked. However, the hydrocracking of asphaltenes as disclosed is at high severity/high conversion, and may present special challenges, as discussed above. For example, operating the asphaltenes hydrocracker at high severity in order to increase the conversion may also cause a high rate of sediment formation, and a high rate of catalyst replacement. In contrast, operating the asphaltenes hydrocracker at low severity will suppress sediment formation, but the per-pass conversion of asphaltenes will be lower.
Processes for upgrading virgin residua hydrocarbon feeds are described in U.S. Pat. No. 8,287,720 which describes hydroprocessing virgin residua in a first reaction unit, solvent deasphalting the effluent, and feeding the deasphalted effluent to a second reaction unit. However, the hydrocracking of residua hydrocarbon feeds and the subsequent process steps are operated at conditions which strain the operating units and produce products having less desirable qualities.
In order to achieve a higher overall partially converted vacuum residua conversion, such processes typically require a high recycle rate of the unreacted partially converted vacuum resid back to one or more of the hydrocracking reactors. Such high-volume recycle can significantly increase the size of the hydrocracking reactor and/or the upstream solvent deasphalting system.